.container {
  height: 300px;
  padding: 0.1px;
  background-color: lightpink;
}
.box_2 {
  height: 200px;
  margin: 0;
  background-color: lightskyblue;
  animation: margin 5s infinite;
  
}

@keyframes margin {
  50% {
    margin: 50px;
  }
}
